3 Physical vs. Logical TopologyThe actual layout of a network and its media is its Physical TopologyThe way in which the data access the medium and transmits packets is the Logical TopologyA glance at a network is not always revealing. Cables emerging from a Hub does not make it necessarily a Star Topology – it may actually be a bus or a ringCP2073 Networking

4 Physical vs. Logical Topology (2)Your choice of Logical Topology will affect the Physical Topology – and vice versaDesign carefully – it may be difficult to change part way through the installationYour choice will determine cable installation, network devices, network connections, protocols (and where you will drill holes in the building !)CP2073 Networking

7 Bus Topology (2) Network maintained by a single cableCable segment must end with a terminatorUses thin coaxial cable (backbones will be thick coaxial cable)Extra stations can be added in a daisy chain mannerCP2073 Networking

8 Bus Topology (3) Standard is IEEE 802.3Thin Ethernet (10Base2) has a maximum segment length of 200mMax no. of connections is 30 devicesFour repeaters may be used to a total cable length of 1000mMax no. of nodes is 150CP2073 Networking

9 Bus Topology (4) Thick Ethernet (10Base5) used for backbonesLimited to 500mMax of 100 nodes per segmentTotal of four repeaters , 2500m, with a total of 488 nodesCP2073 Networking

10 Bus Topology (5) Advantages Inexpensive to installEasy to add stationsUse less cable than other topologiesWorks well for small networksDisadvantagesNo longer recommendedBackbone breaks, whole network downLimited no of devices can be attachedDifficult to isolate problemsSharing same cable slows response ratesCP2073 Networking

12 Ring Topology (2) No beginning or end (a ring in fact !!)All devices of equality of access to mediaSingle ring – data travels in one direction only, guess what a double ring allows !?Each device has to wait its turn to transmitMost common type is Token Ring (IEEE 802.5)A token contains the data, reaches the destination, data extracted, acknowledgement of receipt sent back to transmitting device, removed, empty token passed on for another device to useCP2073 Networking

13 Ring Topology (3) Advantages DisadvantagesData packets travel at great speedNo collisionsEasier to fault findNo terminators requiredDisadvantagesRequires more cable than a busA break in the ring will bring it downNot as common as the bus – less devices availableCP2073 Networking

15 Star Topology (2) Like the spokes of a wheel (without the symmetry)Centre point is a HubSegments meet at the HubEach device needs its own cable to the HubPredominant type of topologyEasy to maintain and expandCP2073 Networking

16 Star Topology (3) Advantages DisadvantagesEasy to add devices as the network expandsOne cable failure does not bring down the entire network (resilience)Hub provides centralised managementEasy to find device and cable problemsCan be upgraded to faster speedsLots of support as it is the most usedDisadvantagesA star network requires more cable than a ring or bus networkFailure of the central hub can bring down the entire networkCosts are higher (installation and equipment) than for most bus networksCP2073 Networking

17 Extended Star TopologyA Star Network which has been expanded to include an additional hub or hubs.CP2073 Networking

19 Mesh Topology (2) Not common on LANsMost often used in WANs to interconnect LANSEach node is connected to every other nodeAllows communication to continue in the event of a break in any one connectionIt is “Fault Tolerant”CP2073 Networking

23 Types of Logical TopologyPrevious slides showed Physical TopologiesOnly two Logical Topologies (Bus or Ring)Physical Bus or Ring easy to conceptualisePhysical Star could be either a Bus or Ring in logical termsConfused ? See next slidesCP2073 Networking

24 Logical Bus Modern Ethernet networks are Star Topologies (physically)The Hub is at the centre, and defines a Star TopologyThe Hub itself uses a Logical Bus Topology internally, to transmit data to all segmentsCP2073 Networking

25 Logical Bus A single node failure does not bring the network downAdvantagesA single node failure does not bring the network downMost widely implemented topologyNetwork can be added to or changed without affecting other stationsDisadvantagesCollisions can occur easilyOnly one device can access the network media at a timeCP2073 Networking

26 Logical Ring Data in a Star Topology can transmit data in a RingThe MAU (Multistation Access Unit) looks like an ordinary Hub, but data is passed internally using a logical ringIt is superior to a Logical Bus Hub – see later slideCP2073 Networking

28 Logical Ring (3) Advantages DisadvantagesThe amount of data that can be carried in a single message is greater than on a logical busThere are no collisionsDisadvantagesA broken ring will stop all transmissionsA device must wait for an empty token to be able to transmitCP2073 Networking